This invention relates to a miniature antenna coil using an induction magnetic field to be utilized in a keyless entry device such as a door handle, a door mirror and the like in a vehicle.
As a conventional keyless entry device in a vehicle, a device in which a remote unit transmits radio waves with a prescribed code when an operator operates a button in a remote unit and locks or unlocks a door of the vehicle and when the code coincides with the specified code registered in the vehicle previously is common. Also, a device in which a door of the vehicle can be unlocked automatically only when a driver carrying a remote unit with him approaches the vehicle without operating a button and a door of the vehicle can be locked automatically when the driver gets out and leaves from the vehicle is known recently in this type of the device.
Referring to FIG. 3, a keyless entry device of the vehicle is provided with a first antenna coil 27 in a remote unit A side, a transmitter 28, and a control circuit 29 which transmits a prescribed signal to the transmitter 28 in response to an electric current from the first antenna coil 27. Radio waves are transmitted between the unit A and the vehicle to lock and unlock the door of the vehicle. Battery 30 supplies electric power to the transmitter 28 and the first control circuit. A second antenna coil 31 and a feeding circuit 32, which supplies electric power to the second antenna coil 31, transmit an electromagnetic wave for activation of the remote unit A, and a receiver 33 which receives the prescribed signal from the transmitter 28 are provided in the vehicle mounting side unit B. The receiver 33 controls the feeding circuit 32 to supply electric power to the second antenna coil 31. When the first antenna coil 27 is in a fixed range to the second antenna coil 31, that is to say, when the scope of magnetic field is within about 1 m from the second antenna coil 31, the electromagnetic wave for activation of the remote unit is coupled by electromagnetic inductive coupling with the first antenna coil 27. The device is also provided with the second control circuit 34, which locks and unlocks the door of the vehicle based on the prescribed signal from the receiver 33 and an automatic locking timer 35 and timer 36, a human body detection sensor 37 in the vehicle and a door lock device driving circuit 38 to drive the door lock device for locking and unlocking the door of the vehicle, which are attached on a portable item for carrying the remote unit A.
In this way, the second control circuit 34 controls the feeding circuit 32 to supply electric power to the second antenna coil 31 and to transmit the electromagnetic wave for activation of the remote unit. The first control circuit 29 activates the transmitter 28 to transmit the prescribed signal based on electric current from the first antenna coil 27, which received the electromagnetic wave for activation to activate the remote unit. And, the second control circuit 34 locks and unlocks the door of the vehicle based on the prescribed signal from the receiver 33 on a vehicle mounting side unit A, which received this prescribed signal. Accordingly, when the first antenna coil 27 is in a fixed range to the second antenna coil 31, the first control circuit 29 receives electric current from the first antenna coil 27 as an activation signal to activate the first control circuit 29 and activates the transmitter 28 for the first time receiving this signal. After the transmitter 28 is activated, a battery 30 in a remote unit A is used only for the stand-by power necessary for stand-by to receive electric power from the first antenna coil 27 in the first control circuit 29 and so the consumption of the battery 30 can be reduced. The keyless entry device is carried by attaching the remote unit A on the portable item. And this keyless entry device is disclosed in a Patent Document 1.
The second antenna coil 31 on the vehicle mounting side is usually attached at the handle of the doorknob in the conventional keyless entry device in the vehicle.
An antenna coil using a bar antenna attached on a door handle is disclosed in a Patent Document 2 as an antenna coil for keyless entry. The disclosed antenna coil has a single core made of thin bar-shaped ferromagnetic substance with a winding. The core and a bobbin wrapping the core are contained in a storing case and an opening between the core and the bobbin and the opening between the bobbin and the case are filled with potting material to prevent damage of the single core unit and deterioration by temperature, humidity and the like.                [Patent Document 1] JP2001-115700 Publication of unexamined patent applications        [Patent Document 2] JP2001-358522 Publication of unexamined patent applications        